This invention relates to transportation, and particularly to the construction of wheels for wheeled vehicles.
The current technology for roller coaster load wheels employs a cast aluminum hub with a thin tire made from a sophisticated polyurethane. The tire is cast on and bonded to the hub. The wheel is mounted to the axle of the roller coaster vehicle through a bearing. A great deal of research and testing has been undertaken over the years to derive the optimal tire material in an attempt to balance the demanding requirements.
In certain applications there are extreme requirements placed upon the wheel assembly of a wheeled vehicle. This is particularly true in amusement parks where high-performance roller coasters today operate at speeds in excess of 80 miles per hour and experience in excess of 5 Gs of acceleration in the maneuvers. On a typical coaster, there as many as 100 wheels of various types. The wheels that carry the weight of the vehicle and its load are called xe2x80x9cThe Load Wheels.xe2x80x9d Other wheels are xe2x80x9cGuide Wheelsxe2x80x9d or xe2x80x9cUpstop Wheels.xe2x80x9d
There are several fundamental requirements on the Load Wheels: They must carry the load of the roller coaster and the occupants. This load can be as high as five times the weight of the vehicle and its occupants (in 5-g maneuvers) divided among the load wheels. This can be in excess of 5000 pounds of force per wheel.
They must be capable of carrying this load while the vehicle is traveling at high speeds. At 80 miles per hour, a 9-inch diameter wheel, a typical size, spins at approximately 3000 revolutions per minute.
They must have low rolling resistance. The dynamic maneuvers are only possible when the losses are minimized. The requirement for low rolling resistance drives the tire toward a harder material. Typical rolling resistance of a roller-coaster wheel is between 0.010 and 0.017 of the load supported.
They must provide a Smooth Ride. Most roller coasters have no compliant element between the track and the occupants except the tire on the wheels. It is very difficult to construct the very large welded track so that it is smooth enough to allow for little or no compliance in the vehicle. As a result, the tires must absorb the small imperfections of the track. Contrary to the rolling resistance requirement, the Smooth Ride requirement drives the tire toward a softer material.
They must provide minimum cyclic maintenance Costs. In particular, the amusement park operators require that the tires last as long as possible and would prefer it to be inexpensive to replace the tires when necessary. These are the basic requirements on the Load Wheels. The following section describes the current state of the art for these wheels.
The current technology for roller coaster load wheels has significant limitations, including the following:
Limited operating Range
The best wheels with the best known tire materials using the current methods are not always capable of handling the range of operation desired by the roller coaster manufacturers and owners. The wheels, as much or more than any other element of the roller coaster system, have set the upper boundary for performance. While modern coasters are being designed to run faster and to have more dynamic maneuvers than ever before, the wheels in use today are falling short of the speed and load ranges demanded of them.
Failure Modes
The following are the failure modes of the current wheels under these dynamic conditions:
Internal Meltdown. The polyurethane tire is constantly being deformed as the wheel rolls down the track under load and at speed. The very localized point where the tire contacts the typical tubular track is compressed significantly. As the wheel rolls, that point re-expands, and the next point is compressed. This constant xe2x80x9ckneadingxe2x80x9d of the material results in a process known as xe2x80x9chysteretic heating.xe2x80x9d Temperatures inside Load Wheels can exceed 250 degrees Fahrenheit. When temperature inside the tire reaches the melting point of the material, the internal urethane liquefies and is ejected out of the sides.
Bond Line Failure. As a result of the elevated stress cycles on the wheel, the bond between the tire and the hub may become weakened. At this point, the tire begins to move independently from the hub. The tire is eventually torn or driven off the hub. Until these problems are solved, roller coaster design is limited by the capabilities of the wheels.
Wear
The current technology roller coaster wheels wear out more frequently than the designers and customers would desire. This is true of both the highly dynamic coasters and the more mild ones as well. When the tires wear out, they need to be replaced. This results in installations that are unavailable for use and in significant maintenance costs. The principal modes for the tires wearing out are: material loss, chunking, and spalling of the polyurethane as a result of cyclic exposure to high loads and track imperfections as well as dirt and debris. While using a harder tire material can significantly reduce this problem, the result is an unacceptably rough ride. Wheel designers have constantly battled with the balance between the requirements for a Smooth Ride and acceptable wear. The best materials in use today on dynamic roller coasters have what many believe is insufficient wear characteristics.
Operating Costs
The problem of excessive wear is compounded by very high maintenance and repair costs. The process of xe2x80x9cre-treadingxe2x80x9d a wheel is very expensive. First, the old material must be carefully removed; then the surface chemically treated to accept the new tire. Finally the new tire is cast onto the hub. The casting and set-up time can run as long as several weeks depending on the material. A significant portion of the ongoing operational cost of a roller coaster is in tire replacement.
According to the present invention a wheel assembly for a wheeled vehicle is provided with separate inner and outer tire members, the outer tire member being made of a relatively hard and rigid material while the inner tire member is made of a relatively softer and more resilient material.
According to the presently preferred form of the invention the outer circumferential surface of the inner tire member and the inner circumferential surface of the outer tire member have interfitting shapes such as to hold them in a fixed relative position, and the inner tire member is sufficiently resilient so that it may be temporarily deformed when fitting it into the outer tire member.
More specifically, a wheel assembly in accordance with the invention includes a generally ring-shaped integrally formed outer tire member having a laterally convex circumferential inner surface, and a generally ring-shaped integrally formed inner tire member that is of substantially uniform thickness throughout its lateral width and has a laterally concave outer surface. The outer tire member is made of relatively hard and relatively rigid material, and the inner tire member is made of resilient material, so that the inner tire member may be deformed in order to position it inside the outer tire member with the concave outer surface of the inner tire member in interfitting relation to the convex inner surface of the outer tire member.
Further according to the present invention, because of the substantially uniform thickness of the inner tire throughout its width, it has a laterally convex inner circumferential surface. A hub assembly is adapted to engage and support the laterally convex inner circumferential surface of the inner tire and to maintain a radially outward force on it, in order to maintain the assembled position of the inner and outer tire members without the use of a separate bonding material or special bonding action between them.
Other features and advantages of the invention will be apparent from the detailed disclosure which follows.